The present invention relates to materials which are useful for crosslinking silazane polymers. These crosslinkers improve the processability of the silazanes so they can be used as precursors to the matrix in ceramic matrix composites.
Ceramic matrix composites (CMCs) and methods for there manufacture are well known in the art. Generally, CMCs comprise a matrix phase which can be formed by chemical vapor deposition, polymer infiltration pyrolysis, and the like and a reinforcement phase such as fibers.
The polymer impregnation approach to making CMCs essentially comprises infiltrating an assembly of fibers with a preceramic polymer and then heating the impregnated fibers to ceramify the polymer. Often, the body formed thereby needs repeated impregnation with the preceramic polymer and pyrolysis to increase the density of the resulting object.
The preparation of thermally and oxidatively stable CMCs by the polymer impregnation approach is at least partially dependent on the preceramic polymer chosen for impregnation of the fibers. Silazane polymers (or polysilazanes--characterized as having backbones with alternating silicon and nitrogen atoms) have been shown in the art to be useful for this purpose.
To be truly useful as ceramic precursors, however, the polysilazanes must be curable (infusible or crosslinkable) to prevent deformation of the ceramic upon heating. Various approaches to providing curability have been suggested. Thus, for instance, European Publication 549,224 teaches the use of silazanes in a polymer impregnation and pyrolysis process for manufacturing CMCs. This patent mentions the desirability of curable polymers but does not teach the curing materials set forth herein.
U.S. Pat. No. 5,086,126 teaches a method of producing silazane polymers having functional groups attached thereto. The method described therein comprises treating a silazane polymer having N--H bonds with an alkyllithium and then reacting the resultant material with a chlorosilane having the desired functional group. The reference teaches that the functional group may be one which allows for curing the polymer, but it does not teach the curing materials of the present invention.
U.S. Pat. No. 5,169,908 teaches curable hydridopolysilazane polymers obtained by reacting these polymers with borane. The resultant polymers have B--H bonds which, upon heating, can react with Si--H and N--H bonds in the polymers and, thus, cause them to crosslink. This reference, too, fails to teach the curing materials claimed herein.
U.S. Pat. No. 5,262,553 teaches a method for crosslinking polysilazanes. The method described therein comprises reacting a crosslinking material having 2 boron functional groups with a polysilazane having Si--H or N--H bonds. Pursuant to this reaction, B--N or B--Si bonds are formed and the polymer is thereby crosslinked. Again, this reference does not teach the materials claimed herein.
The present inventors have now discovered that cyclic siloxanes and silazanes can be used to cure Si--H functional silazane polymers.